Power amplifiers are widely used in communication systems, for example in cellular communication systems and cellular base stations wherein high frequency communication signals are amplified for transmission.
Bandwidth and efficiency are important considerations in the design of power amplifiers. In the context of cellular base stations, there is a growing need for improved power amplifier (PA) efficiency. There is also need for reconfigurable cellular base station transmitters due to the growing number of standards and the need for backward compatibility without compromising the overall power amplifier efficiency and linearity. A conventional power amplifier, such as a class-B amplifier, generally provides maximum efficiency at or near its maximum saturated power output level. In order to accurately reproduce a signal of varying amplitude, the peak output signal level should be equal to or less than that maximum saturated power level. When the instantaneous signal output level is less than the peak output level, a conventional class-B power amplifier generally operates at less than maximum efficiency.
More recent cellular communication standards, such as UMTS (Universal Mobile Telecommunication Standard) and LTE (Long-Term Evolution) created within 3 GPP (3rd Generation Partnership Project), use complex modulation schemes whose amplitude component creates large variations in the instantaneous carrier output power of a transmitter. The ratio of the peak carrier output power to the average output power (defined as the “crest factor”) when expressed in decibels (dB), may reach values on the order of 10 dB. With crest factors of such magnitude, efficiency of a base-station power amplifier is severely reduced; in order to be able to process large peak carrier powers, a conventional, linear PA operates several dB below its maximum output power capability (e.g., several dB into back-off) for most of its operational time.
Various approaches have been proposed to address the issue noted above. In one approach, a modified out-phasing technique by Chireix (sold under the brand name “Ampliphase” by RCA) has been proposed. The term “out-phasing” relates to a method of obtaining amplitude modulation (AM) by combining several (generally two) phase-modulated constant-amplitude signals as further described below. These signals are produced in a “signal component separator” (SCS) and subsequently, after up-conversion and amplification through RF chains (mixers filters and amplifiers), combined to form an amplified linear signal within an output combiner network. The phases of these constant-amplitude signals are chosen so that the result from their vector-summation yields the desired amplitude, as the amplitude modulation is achieved by the degree of addition or subtraction due to the phase difference between the two signals.
In the Chireix approach, as noted above, a low-level copy of an intended output signal is resolved into two equal-amplitude components with a phase separation determined by the instantaneous amplitude of the intended output signal. These two equal-amplitude components are then amplified by a pair of RF power amplifiers operating in saturation or switched-mode for optimum power efficiency. The outputs of both the power amplifiers are then combined in a low-loss Chireix combiner so as to re-construct a fully-modulated RF carrier. By so doing, effectively, the resistive load impedance that is seen by both PAs becomes a function of the output phase angle and results in an envelope modulation of the output power expressed as:
                                                                  P              OUT                        ⁡                          (              t              )                                                ∝                              V            DD            2                                2            ⁢                          R              ⁡                              (                                  θ                  ⁡                                      (                    t                    )                                                  )                                                    ∝                                            V              DD              2                                      2              ⁢                              R                L                                              ⁢                                    cos              2                        ⁡                          (                              θ                ⁡                                  (                  t                  )                                            )                                                          (        1        )            
In the Chireix approach described above, benefits are derived from the use of switched-mode rather than linear-mode power amplifiers.
Conventional Chireix out-phasing PA has drawbacks in terms of bandwidth and efficiency. Frequency limitations are imposed by power combiner (e.g., quarter wavelength transmission lines) and fixed susceptance compensation elements (±jBC, as discussed below). Another drawback in the conventional Chireix PA is that it is usually built from saturated linear PAs (e.g., class AB) or harmonically tuned PAs (e.g., class-F), which ideally fail to provide 100% efficiency without making use of harmonic traps in a matching network.
Other approaches based on class-E and other switch mode (e.g., class DE) based out-phasing have other deficiencies, as they have failed to consider integration, reported to have wide RF bandwidth, and failed to account for reconfigurability aspects. Further, other approaches, like the n-way Doherty PA, in general require more tunable elements due to the use of several quarter-wave transmissions lines.